Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Coal processing plants typically employ one or more minerals separation flotation cells to process fine coal. A slurry or pulp containing coal and gangue is fed into the flotation cell or cells, which separate the coal values or concentrates from the gangue by inducing bubbles so that the values float to the surface of the pulp. The coal values are recovered by an overflow weir or launder as a product stream, whereas the gangue is drained from the lower part of the cell as a tailings stream. To facilitate separation and improve the productivity of the floatation cells, typically surfactants and wetting agents are used as “frother” to encourage the production of bubbles or “froth” for transporting the coal values to the pulp surface. An example of a frother is methyl isobutyl carbinol (MIBC).
Whilst frother assists in the flotation process, it acts as a contaminant in the tailings stream and poses an environmental pollutant if it is discharged into the tailings dam. Furthermore, the tailings stream is often recycled to obtain clarified water for use in the pumping system of the rest of the processing plant. The presence of too much frother in the tailings stream contaminates the clarified water, since excess frother causes cavitations in the pumping system and “froths out” the coal processing plant. Flushing the frother out of the pumping system requires the shut down of the entire plant and the associated down time spent in cleaning out the pump system results in wasted labour and lost production.
This problem is exacerbated by the presence of other substances in the tailings stream, such as dissolved salts and some solids, which also facilitate the formation of froth in the liquid.
In this context, coal separation by flotation typically involves only 20% of the coal produced by the entire coal processing plant and is limited to the recovery of fine coal, whereas the rest of the plant produces the remaining 80% of the coal. Consequently, to avoid these potential production losses and environmental hazards, it has been the practice to use substantially reduced amounts of frother in the flotation cells to minimize the presence of frother in the tailings stream. This, however, reduces the efficiency of the flotation cells in recovering fine coal.